1. Field of the Invention
The invention relates to a signal processing apparatus, and more particularly a signal processing apparatus and a method used in an optical disk apparatus for converting a focus-error signal, track-error signal or the like into digital data.
2. Description of the Related Art
Optical disk apparatus require several control systems in order to precisely record or reproduce data. One of them is a focusing control system. The focusing control is to detect the focusing error caused by the surface deflection or up-down movement of the disk surface while the optical disk is rotating, and move an optical system such as an objective lens in parallel with the optical axis of the laser beam according to the amount of detected focusing error so as to accomplish optical focusing. In order to detect the focusing error, for example, a pair of photosensors are used to derive a difference between two outputs therefrom. Then, current corresponding to the signal difference is supplied to an objective lens driving coil to set the objective lens into the correct focusing position (focused position). That is, the laser beam is focused on the optical disk by use of a condenser lens, the reflected light from the optical disk is applied to the pair of photosensors, and then focusing control is effected to reduce the difference between the two outputs from the photosensors to zero.
In general, in the data recording operation, the laser beam output is set larger than in the data reproducing operation, and therefore the amount of light applied to the pair of photosensors becomes large, causing a larger photosensor output current to be supplied. As a result, the loop gain of a focusing controller becomes large. Conventionally, in order to keep the loop gain of the focusing controller at a constant value and stabilize the servo system, an analog divider or digital divider is used to normalize the photosensor output.
In a case where the analog divider is used, a difference signal of two output signals is divided (normalize) by a sum signal thereof, the result is subjected to A/D conversion process, and then the focusing control operation is effected according to digital data obtained in the A/D conversion process. In contrast, when the digital divider is used, a difference signal of two output signals and a sum signal thereof are subjected to A/D conversion process, respectively, and then the focusing control operation is effected according to digital data obtained by dividing the A/D converted data by use of the digital divider.
However, when the analog or digital divider is used, the focusing controller will become large in size and extremely expensive. In particular, when the digital divider is used, it is necessary to provide a high reference voltage in order to attain precise A/D conversion when a larger amount of light is applied to the photosensors. In contrast, when the amount of incident light is small, the reference voltage becomes low and the resolution for the A/D conversion is lowered. With the fact described above taken into consideration, it has been proposed to provide a device which can derive out precise digital data.